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Serious measure — key workers are in the dark on this (9,5)

Ross

I believe the answer is:

graveyard shift

'workers are in the dark on this' is the definition.
(the graveyard shift is a shift done at night time)

'serious measure key' is the wordplay.
'serious' becomes 'grave' (both can mean solemn).
'measure' becomes 'yard' (yard is a kind of measure).
'key' becomes 'shift' (key on a computer keyboard).
'grave'+'yard'+'shift'='GRAVEYARD SHIFT'

(Another definition for graveyard shift that I've seen is " work so late".)

I've seen this clue in The Guardian.
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